Nearly 650 New Zealanders snapped up super cheap airfares as low as $NZ50 ($A40) for flights to Europe in September after a data inputting error on the website of KLM Royal Dutch Airlines, a report says.

But the lucky travellers may have to fork out more money for their flights after KLM cancelled the bookings and began issuing refunds.

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A watchdog - the NZ Commerce Commission - rebuked the airline for taking more than four hours to correct the mistake and said it should have better systems in place.

KLM advised the travel industry on September 25 that it was increasing its airfares by $NZ50 ($A40) for economy class flights and $NZ100 ($A80) for business class, Adrian Sparrow, the commission's fair trading director, said in a statement.

"However, a data inputting error in Amsterdam resulted in the intended price increases becoming the total fare prices on the KLM website and travel fare databases."

Sparrow said that when the error was discovered, KLM notified the travel industry of the mistake and subsequently cancelled bookings and began issuing refunds.

"However, during the several hours it took for the problem to be rectified, KLM sold 649 tickets through its website at the incorrect prices."

Sparrow said there was evidence that many who bought tickets were connected with the travel industry and made bookings after the mistake had been announced.

Many travellers also knew or suspected that they were taking advantage of an error, he said.

Sparrow said the commission was not prosecuting KLM, but consumers could take private legal action under the Fair Trading Act or through the Disputes Tribunal.